In order to control the flight and other operational characteristics of an unmanned vehicle, such as a satellite, a missile, or a re-entry vehicle, a conventional satellite includes a significant amount of control electronics. Among other things, the control electronics control the flight of the satellite by selectively activating the rockets that propel and direct the satellite toward its orbit. In this regard, the control electronics generally control valves associated with each engine which are designed selectively to provide fuel to the engines. In addition, the control electronics direct the operation of a number of sensors and other instruments which aid in the guidance of the satellite as well as the operation of communications equipment. Additionally, the control electronics typically activates the batteries associated with the on-board electrical equipment, such as sensors, instruments, communications equipment and the like, in order to activate the on-board electrical equipment. Likewise, the control electronics typically squib the pressure tanks in order to build pressure in the fuel tank and the oxidizer tank.
Conventionally, the control circuitry of satellites or other unmanned vehicles includes separate driver circuits, each of which typically includes a separate controller, for providing control signals to respective electrical subsystems. For example, separate driver circuits are generally associated with each valve in order to control the associated engines. In this regard, a valve driver circuit would typically provide signals to the solenoid of the respective valve that cause the valve to open or close. In instances in which the valve is open, a mixture of fuel is typically provided to the engine which causes the engine to propel the satellite in a desired direction. Alternatively, in instances in which the valve is closed, fuel is no longer provided to the engine and no further propulsion is provided by the engine. Since more current is generally required in order to initially open a valve as opposed to maintaining a valve in an open position, the control electronics associated with the valve of each engine preferably initially provides a pull-in current in order to open the valve and then subsequently provides a hold current, which is significantly less than the pull-in current, in order to maintain the valve in the open position.
In addition to the separate driver circuits that are typically required for the solenoid of each valve, separate driver circuits are typically required for actuating each ordnance or squib carried by the satellite. In this regard, ordnances or squibs are typically associated with batteries carried by the satellite such that the actuation of an ordnance activates the respective battery which, in turn, provides power to other associated electrical subsystems, such as a sensor, an instrument, communications equipment or the like. In addition, ordnances or squibs can be disposed between the pressure tanks and the fuel tank and the oxidizer tank in order to cause the fuel and oxidizer tanks to be pressurized upon actuating of the ordnances.
However, since separate driver circuits are typically provided for each valve and each ordnance, the control electronics of a conventional unmanned vehicle is unfortunately relatively heavy and occupies a significant amount of space. In order to reduce the weight of the control electronics and to reduce the space consumed by the control electronics, a multi-channel driver circuit has been developed for controlling a plurality of valves and a plurality of ordnances. In this regard, the multi-channel driver circuit includes a plurality of valve drivers associated with respective valves and operating under control of a common controller, such as a programmable logic device. As such, the multi-channel driver circuit can individually actuate each valve, in order to open the valve and provide fuel to the respective engine. In particular, each valve driver can provide a pull-in current for a predetermined pull-in time in order to initially open a valve and can then provide a reduced level of current, namely, a hold current, for the remainder of the period during which the valve is held in an open position. A multi-channel driver circuit is described in U.S. Pat. No. 6,267,326 to Smith et al., which is incorporated herein by reference in its entirety.
U.S. Pat. No. 6,267,326 to Smith et al. also discloses one method of applying the correct pull-in and hold currents to the valves. In U.S. Pat. No. 6,267,326 to Smith et al., a comparator is used to measure the current applied to a valve. The comparator compares the voltage appearing across a relatively small resistor to a predetermined value and provides the controller with an indication of whether the voltage across the resistor is greater or less than the predetermined amount which, in turn, provides an indication that the current flowing through the valve is greater or less than desired. If less current is flowing through the valve than desired as indicated by the output of the comparator, the controller directs that the valve remains connected to the supply voltage and current flows through the valve. If, however, the current provided to the valve is greater than desired as indicated by the voltage appearing across the relatively small resistor being greater than the predetermined value, the comparator provides the controller with a feedback signal such that the controller, in turn, disconnects the valve from the supply voltage. Once the comparator detects that the voltage across the resistor falls below the predetermined level, the comparator provides the controller with another signal which causes the controller to reconnect the valve to the supply voltage and to have current pass therethrough. As such, the current actually provided to the valve generally oscillates about the desired value as the valve is alternatively disconnected and connected to the supply voltage as the voltage appearing across the resistor exceeds and then falls below the predetermined value, respectively. The predetermined voltage level is generally set by a voltage divider network, with the particular resistor values predetermined to establish the predetermined voltage levels. In this method, the pull-in and the hold currents cannot be easily modified, as the resistor values of the voltage divider network must be modified to change the reference voltage value, which would in turn change the pull-in and/or hold currents.
The pull-in and hold currents may need to be modified by the manufacturer of the satellite. For example, during testing of the satellite it may be determined that a larger pull-in current is required. Alternatively, the valve may need to be replaced with a different valve with different pull-in and hold current requirements. The requirement to physically change the circuitry in order to change the pull-in and/or hold currents can be very costly and time consuming, especially when the changes are made late in the design development of the satellite. Additionally, conditions that occur during flight of the satellite, such as changes in valve resistance and inductance, may alter the pull-in and/or hold current requirements. Physical changes to the circuitry are not possible during flight.
Therefore it would be desirable to have an improved driver for solenoid valves that permit the necessary parameters to open and maintain solenoid valves to be quickly and easily modified, thereby allowing design changes to the satellite to occur late in the development cycle, or allowing changes to the pull-in and/or hold currents to be made during flight.